A navigation system provides travel guidance for enabling a user to easily and quickly reach the selected destination. A typical example is a vehicle navigation system in which a vehicle is equipped with a navigation function to guide a driver to a destination through a calculated route. Such a navigation system detects the position of the user's vehicle, and reads out map data pertaining to an area at the current vehicle position from a data storage medium. Typically, the navigation apparatus displays a map image on a monitor screen while superimposing thereon a mark representing the current vehicle position and a calculated route to the destination. At an intersection associated with the calculated route, the navigation apparatus notifies the user which direction to turn at the intersection to reach the destination.
FIG. 1 shows an example of map image displayed on a monitor screen of the navigation system which shows a map of a congested area such as a downtown of a city. Typically, the navigation system displays a street on which the vehicle (vehicle position VP) is running on a map image and a name of the street. Other information such as a north pointer, a cursor at the center, a map scale, a current time, etc. may also be illustrated on the display screen.
In the example of FIG. 1, buildings and other landmarks are illustrated in a three-dimensional manner in the map image to give the driver an intuitive view of the surrounding area in the city. In addition to the roads and buildings, the, map image includes a large number of POI icons indicating points of interest such as restaurants, ATMs, gas stations, and some other footprints of structure. The roads include large scale roads such as interstate freeways, and small scale roads such as local roads.
As can be seen from FIG. 1, the map image of downtown area includes so many representations of map elements, i.e., different icons (building images, POI icons, traffic icons, etc.) and colors. As a result, too many colors make more important information, such as the current vehicle position, road network and traffic condition less visible. If the driver tries to read or otherwise detect desired information from the complicated map image on the monitor screen, it may distract his attention from the safe driving.
FIGS. 2A-2B and 3A-3B are examples of map image on the screen showing different road widths and colors depending on their function (road) classes specified in a map database. These examples show side-by-side comparison of map images for an urban area (downtown area) with an open area (local area) in the conventional technology. The display method of FIGS. 2A-2B and 3A-3B visually allows the user to confirm what kind of street where he/she is on, especially in a downtown area where several classes of roads exist in a relatively small area.
For example, a freeway is displayed in a symbol having a 5-pixel width in red color; a major surface street is displayed in a symbol having a 5-pixel width in brown color; a surface street is displayed in a symbol having a 4-pixel width in white color; and a minor surface street is displayed in a symbol having a 2-pixel width in light gray color, and the like. According to this method, freeways are more visible than minor surface streets as shown in FIGS. 2A and 3A. However, when this method is applied to an open area such as a country side in the same scale and color as illustrated in FIGS. 2B and 3B, the driver may not really be able to see the minor streets. It may give a feeling that he/she is driving in a desert.
In order to solve the problems described above, several attempts have been made. Japanese Utility Model No. H7-29410 discloses the method for valuing the landmarks on map data by determining the number of the landmarks on a screen and limiting the number of landmarks to be displayed when the number of landmarks is equal to or more than a predetermined number. However, because this method simply limits the landmarks by the number, sometimes it may have to limit important landmarks.
Japanese Patent Application Laid-Open No. 2006-3005 discloses a navigation apparatus having display control means for displaying facility icons, such as restaurants, drug stores and gasoline stations on map data by stop displaying the facility icons when the number of facility icons are more than a predetermined umber. Again, because this method simply limits the facility icons by the number, important facility icons have to be eliminated even though it is important for the user.
International Application Publication No. WO30/107312 discloses a method for representing a selected map extract on a display of a driver information system. A color or a gray value of the map information, especially of a road, is adapted to the color of the map background with a limited maximum color contrast. However, when the number of icons displayed on a screen increases, many icons on the screen make the vehicle position and road network less visible.